OCT is an imaging technology for dividing the light output from a light source into signal light and reference light, detecting interference light obtained by superimposing the signal light having passed through an object with the reference light, and forming a tomographic image of the object based on the detection result. OCT includes a time domain method, spectral domain method, swept source method, etc. Time domain OCT is a tomographic imaging technology that acquires an A-line by sequentially scanning and detecting the pixels in the depth direction of the object. Spectral domain OCT is a tomographic imaging technology that acquires an A-line by sequentially resolving and detecting the spectrogram of the interference light. Swept source OCT is a tomographic imaging technology that acquires an A-line by using a light source whose output wavelength varies sequentially. In swept source OCT, the spectrogram of the interference light, which corresponds to the A-line, is acquired by detecting the interference light sequentially as the wavelength of the light source varies.
For OCT, the selection of wavelength and bandwidth of the light source is important. For example, the wavelength affects the penetration depth of the light into the object, while the bandwidth of the light source (wavelength range) affects the axial resolution (depth resolution). It should be noted that the value of the wavelength is represented by the value of a representative wavelength within the wavelength range to be used. In addition, the definitions of the representative wavelength and the wavelength range both depend on the measurement method used in the OCT system. For example, in an OCT system that employs the time domain system or spectral domain OCT method, a broadband light source such as a super luminescent diode (SLD) is used, wherein as a representative wavelength, a center wavelength of the light source is generally used, and as a wavelength range, a half width (full width at half maximum, FWHM) of the light source is generally used. On the other hand, in a swept source OCT system, a wavelength-sweeping light source is used. Therefore, as a wavelength range, such as a wavelength sweeping range of the light source, a range of spectrum of the light source to be detected by a detector, or a range of spectrum of an interference signal to be provided to the imaging processing, is used. In addition, as a representative wavelength of the swept source OCT system, a wavelength with a maximum intensity within the wavelength range is generally specified.
In addition, some media having light absorbing characteristics affect OCT measurement. For example, in the case of carrying out the OCT measurement of a human fundus, the measurement efficiency becomes low when the light is absorbed by a medium in the eye (particularly, water) and the spectrum is deformed by the absorption characteristics of the medium. According to the conventional OCT technology, this adverse effect is avoided by limiting the wavelength range of the light source used for the measurement to a wavelength region in which the absorption of light by the medium is small. For example, U.S. Pat. No. 7,450,242 discloses a technology, which, by taking into consideration the light loss due to water absorption and the two peaks of water absorption in the vicinity of a 1-μm wavelength (refer to FIG. 1), limits the useful wavelength range (wavelength sweeping range) to 0.98-1.20 μm. In addition, in the publication by B. Pova{hacek over (z)}ay et al., “Enhanced visualization of choroidal vessels using ultrahigh resolution ophthalmic OCT at 1050 nm, 25 Aug. 2003/Vol. 11, No. 17/OPTICS EXPRESS pp. 1980-1986”, for similar reasons, a light source with a center wavelength of 1050 nm and a FWHM of 165 nm was used.